The effectiveness of antenatal intervention in women with increasing obesity is unknown. This study investigated whether there was a differential effect of antenatal intervention on diet, physical activity and pregnancy outcomes in women stratified by obesity class using data from the UK Pregnancies Better Eating and Activity Trial (UPBEAT) ( = 1555). The stratification was by World Health Organization classifications: Class I, II and III (30-34.9 kg/m, 35-39.9 kg/m and ≥40 kg/m). Using linear and logistic regression, adjusted for confounders, outcomes were assessed post-intervention (27-28 weeks' gestation) and in late pregnancy (34-36 weeks' gestation). Interactions between obesity class and the intervention were explored. Compared to the standard care arm, class III intervention women had lower gestational weight gain (GWG) (-1.87 kg; 95% CI -3.29 to -0.47, = 0.009), and the effect of the intervention was greater in class III compared to class I, by -2.01 kg (95% CI -3.45 to -0.57, = 0.006). Class I and II intervention women reported significantly lower dietary glycaemic load and saturated fat intake across their pregnancy. This differential effect of the intervention suggests antenatal interventions for women with obesity should stratify outcomes by obesity severity. This would inform evidence-based antenatal strategies for high-risk groups, including women with a BMI ≥ 40 kg/m.